my $count = Core::Models->resultset('Shipment')->search(
  undef
)->count();

##</code><code>##

my $count = Core::Models->resultset('Shipment')->search(
  { 'rma.id' => {'!=' => undef} },
  { prefetch => 'rma' }
)->count();

##</code><code>##

SELECT COUNT( * ) FROM (SELECT me.id FROM shipment me LEFT JOIN rma rma ON ( rma.shipment_reference =  me.ref AND rma.merchant_id = me.api_merchant_id ) WHERE ( rma.id IS NOT NULL ) GROUP BY me.id) me